C++11: what's the difference between atomic<T>::store and atomic_store<T>

Question

One is a member function of template class std::atomic, one is template function, seems they do the same thing. As std is a class library, why it provides both class and none-class version of, I think the same operation?

Is there any real differences between them?

Answer 1

There's no difference in semantics. The free functions were an attempt to achieve source compatibility with C11:

#ifdef __cplusplus
#include <atomic>
#define _Atomic(X) std::atomic<X>
#else
#include <stdatomic.h>
#endif

_Atomic(int) c;

int get_c(void) { 
    return atomic_load(&c); 
}

Answer 2

Just like you said - one is a class, another is a function. Class have the interface - atomic<T> would provide for stores, loads, proper constructors, etc.

On the other hand, atomic_store could be specialized for your types.